1. Field of the Invention
The present invention relates to an image bearing member and an image forming apparatus using the image bearing member.
2. Discussion of the Background
In recent years, an organic photoconductor (OPC) has superseded an inorganic photoconductor in a photocopier, a facsimile machine, a laser printer and their multi-functional device as an image bearing member in light of performance and advantages. For examples, such an organic photoconductor has the following advantages over an inorganic photoconductor: (1) a wide light absorption wavelength range and a large light absorption amount with regard to optical characteristics; (2) high sensitivity and stable chargeability with regard to electric characteristics; (3) wide selection range of material; (4) easiness of manufacturing; (5) low cost; and (6) non-toxic.
On the other hand, along with the advance of size reduction of an image forming apparatus, the image bearing member therein has been also reduced in size. Furthermore, due to movement toward high speed performance and maintenance-free, an image bearing member having a high durability has been strongly demanded. In light of this, an organic photoconductor is generally soft considering that the main components of the surface layer thereof are a low molecular weight charge transport material and an inactive polymer. Therefore, when such an organic photoconductor is repetitively used in the electrophotographic process, the organic photoconductor has a disadvantage of being easily abraded under the mechanical stress caused by a development system or a cleaning system. In addition, to meet the demand of producing quality images, the size of toner particles decreases, which requires improvement of cleaning property. Thus, the hardness of rubber of a cleaning blade and the contact pressure thereof to an image bearing member increases. This results in another factor of accelerating the abrasion of the surface of an image bearing member. Such abrasion of an image bearing member invites deterioration of electric characteristics such as sensitivity and chargeability and causes reduction of image density, background fouling, etc., which leads to production of abnormal images. When such abrasion occurs locally and damages the surface of an image bearing member, obtained images have streak fouling. In the current situation, the life of an image bearing member is limited by this abrasion or scar, meaning that replacement is necessary.
With regard to improvement on the durability of an organic photoconductor (image bearing member), reducing the amount of the abrasion described above is inevitable and an imminent problem to be solved in this field.
There are following technologies for improving the anti-abrasion property of a photosensitive layer: (1) using a surface layer containing a curing binder resin {refer to unexamined published Japanese patent application No. (hereinafter referred to as JOP) S56-48637}; (2) using a charge transfer polymer (refer to JOP S64-1728); and (3) using a surface layer in which an inorganic filler is dispersed (refer to JOP H04-281461). Among these technologies, the technology of (1) tends to result in image density reduction since the curing binder resin is poorly compatible with a charge transport material or the residual voltage increases due to impurities such as a polymerization initiator or non-reacted residual groups. With regard to the technologies of (2) and (3), it is possible to improve anti-abrasion property in some degree but not satisfactorily. Furthermore, in the technology of (3), the residual voltage rises due to trap existing on the surface of the inorganic filler so that the image density tends to decrease. In conclusion, the technologies of (1), (2) and (3) have not sufficiently satisfied the total durability including electric durability and mechanical durability demanded for an organic photoconductor.
Furthermore, in relation with the technology of (1), an image bearing member is known which contains a curing type acrylate monomer having multiple functional groups to improve anti-abrasion property and anti-damage property {refer to Japanese patent No. (hereinafter referred to as JP) 3262488}. Although there is a description that this curing type acrylate monomer having multiple functional groups can be contained in the protective layer formed on a photosensitive layer with regard to this image bearing member, there is no specific description about the monomer. Furthermore, when a low molecular weight charge transport material is simply contained in the surface (protective) layer, a compatibility problem arises with the curing type material mentioned above. This causes precipitation of the low molecular weight charge transport material, cracking, and deterioration of mechanical strength of the layer. There is also a description about containing a polycarbonate resin to improve the compatibility, but the content of the curing type acryl monomer decreases so that sufficient anti-abrasion property is not obtained. With regard to an image bearing member which does not contain a charge transport material in the surface layer thereof, There is also another description that the surface layer is made to be thin to lower the voltage at irradiated portions. However, when the surface layer is thin, the life of an image bearing member is short and actually it is not possible to maintain an effective value with regard to the voltage at irradiated portions.
There is known a technology improving the anti-abrasion property of a photosensitive layer superseding the technologies mentioned above (refer to JP 3194392). In this technology, a charge transport layer is provided which is formed by a liquid application containing a monomer having a carbon-carbon double bond, a charge transport material having a carbon-carbon double bond and a binder resin. This binder resin is a binder resin having a carbon-carbon double bond and reactivity with the charge transport material mentioned above and a binder resin having no carbon-carbon double bond and no reactivity with the charge transport material mentioned above. This image bearing member is notable in terms of good combination of anti-abrasion property and electric characteristics. When a binder resin having no reactivity with the charge transport material mentioned above is used, the compatibility between the binder resin and the cured material produced from the reaction between the monomer mentioned above and the charge transport material is bad. Therefore, a rough surface is made during cross-linked due to the phase separation, which tends to degrade cleaning performance. In addition that the binder resin interferes curing of the monomer as described above, the monomer specified for use in this image bearing member has two functional groups, which is too small in number to obtain sufficient cross linking density. Thus, the obtained image bearing member has an insufficient anti-abrasion property. When a binder resin having reactivity with the charge transport material mentioned above is used, the number of functional groups contained in the monomer mentioned above and the binder resin is too few to have a good combination of the combined amount of the charge transport material mentioned above and the cross-linking density. Thus, the electric characteristics and the ant-abrasion property are not sufficient.
There is also known a photosensitive layer having a compound formed by curing a positive hole transport material having at least 2 chain reaction polymerizable functional groups in a same molecule (refer to JOP2000-66425). However, since the positive hole transport material is bulky and has at least 2 chain reaction polymerizable functional groups, there is distortion in the cured material, which increases internal stress in the photosensitive layer. This internal stress tends to make the surface layer rough and cause cracking, meaning that the durability is not sufficient.
As a method of solving the problems mentioned above about the anti-abrasion property, electric characteristics, surface property and cracking, JOP 2004-302451 describes an image bearing member having a cross-linking type charge transport layer which is provided on a charge transport layer, has a layer thickness of from 1 to 10 μm and is formed by curing at least a monomer having at least three radical polymerizable functional groups without a charge transport structure and a monomer having one radical polymerizable functional group with a charge transport structure. This image bearing member has a good anti-abrasion property and maintains stable electric characteristics in comparison with the image bearing members because the provided specific cross-linking type charge transport layer significantly improves the durability of the image bearing member. However, as the processing speed of an information processing terminal have increased in recent years, the technologies dealing with a large amount of information in a short time advance day by day. At the same time, an image forming apparatus is demanded to increase the output speed and be durable for an extended period of time, for example, at least 1 million image impressions on sheets. Actually, the image bearing member described in JOP 2004-302451 has a problem that abnormal images are produced due to deterioration of electric characteristics when the image bearing member is used for an extended period of time, for example, 1 million or more image impressions on sheets.
Such abnormal images produced in such a repetitive use over a long period of time are caused by a residual image due to charges having a reverse polarity generated while a toner image is transferred to a transfer member. Especially, this abnormal image is significantly generated in an image forming apparatus taking a direct transfer system. As the method of solving this problem of abnormal images in this direct transfer system, JOP 2006-227496 describes an image bearing member in which a charge blocking layer, a moiré prevention layer, a photosensitive layer and a protective layer are accumulated on an electroconductive substrate and the protective layer is formed by curing at least a monomer having at least three radical polymerizable functional groups without a charge transport structure and a monomer having one radical polymerizable functional group with a charge transport structure as described in JOP 2004-302451.
The image bearing member according to the technology described in JOP 2006-227496 has relatively good durability in comparison with that described in JOP 2004-302451. However, the residual image mentioned above is still observed when the image bearing member is used for an extended period of time, for example, 1 million or more image impressions on sheets. As a result, the image bearing member provided by the technology does not sufficiently have a mechanical durability and stable electric characteristics for forming quality images for an extended period of time.